1. Field of the Invention
The present invention pertains to a method for plasma etching deep trenches, typically at least 50 xcexcm deep, into a silicon substrate while maintaining control over the sidewall profiles which border the trench.
2. Brief Description of the Background Art
In semiconductor device fabrication, it is frequently desirable to create deep trenches in a silicon substrate. Typically silicon dioxide has been used as a masking material for plasma etching of trenches in an underlying silicon substrate. The plasma etch selectivity of silicon relative to silicon oxide, i.e. the ratio of the silicon etch rate to the silicon oxide etch rate, is up to about 40:1, depending on the composition of the plasma etchant. The trench is typically etched in a silicon substrate using a plasma feed gas comprising chlorine, or oxygen, or a combination thereof. The plasma feed gas may include small amounts of SF6, added for profile control.
While 40:1 selectivity is acceptable for a number of fabrication applications, such as gate type field effect transistors, it is not adequate for many micromachining applications, or for fabrication of deep trench capacitors, deep trench isolation (DTI) for high frequency circuitry, power devices, and numerous other applications. For example, with respect to especially deep trenches of the kind required for deep trench capacitors used in DRAM cells where the trench may be 200 xcexcm deep, a 40:1 selectivity would require a silicon oxide mask thickness of about 5 xcexcm, which causes numerous problems. Use of such a thick masking layer of silicon oxide makes it more difficult to control the sidewall profile of the trench as it is etched; since, as etching progresses the edges of the masking layer above the opening are eroded away and the shape at the top of the opening changes.
It is highly desirable to have a thin masking layer which has a sufficiently low etch rate compared to that of silicon that the selectivity is extended beyond 40:1.
The present invention provides a method for etching trenches, contact vias, or similar features to a depth of 100 xcexcm and greater while permitting control of the etch profile (the shape of the sidewalls surrounding the etched opening). The method requires the use of a metal-comprising masking material in combination with a fluorine-comprising plasma etchant. The byproduct produced by a combination of the metal with reactive fluorine species must be essentially non-volatile under etch process conditions, and sufficiently non-corrosive to features on the substrate being etched, that the device features remain unharmed by the etch process. By unharmed it is meant that performance of a semiconductor device feature in the substrate is not affected, or that any residue from the etch process that could affect feature performance can be removed to leave the device feature essentially unaffected.
Although aluminum is a preferred metal for the metal-comprising mask, since aluminum is already present in most semiconductor processing chambers and the effect produced by the presence of aluminum in most semiconductor processes is known, other metals can be used for the masking material, so long as they produce an essentially non-volatile, non-corrosive etch byproduct under etch process conditions. By way of example, and not by way of limitation, metallic materials recommended for the mask include aluminum, cadmium, copper, chromium, gallium, indium, iron, magnesium, manganese, nickel, and combinations thereof. In particular, aluminum in combination with copper or magnesium is particularly useful, where the copper or magnesium content is less than about 8% by weight, and other constituents total less than about 2% by weight.
The plasma feed gas includes at least one fluorine-containing compound such as nitrogen trifluoride (NF3), carbon tetrafluoride (CF4), sulfur hexafluoride (SF6), silicon tetrafluoride (SiF4), trifluoro methane (CHF3), difluoro methane (CH2F2) and perfluoro 1-butene, or perfluoro 2-butene, or perfluro cyclobutane (C4F8), by way of example and not by way of limitation.
Oxygen, or an oxygen-comprising compound may be added to the plasma feed gases to help provide a protective layer over etched sidewalls, assisting in profile control of the etched feature. The preferred atomic ratio of oxygen:fluorine in the plasma feed gas ranges from about 0.25 to about 0.50. Plasma feed gases comprising CH2F2 and SF6, and O2; or comprising C4F8 and SF6, and O2 work well, for example and not by way of limitation.
The addition of hydrogen bromide (HBr) to the plasma feed gas also helps protect sidewall integrity. And, although the non-HBr chemistries mentioned above work well, they are compound depositing in nature, so that a combination of HBr with SF6 and O2, for example provides a cleaner process and is preferred.